Green lumber is typically stacked, grouped in batches, and dried batch-wise in a kiln. The batches of lumber (“charges”) are placed within an insulated chamber in the kiln, the chamber is closed, and conditions within the chamber (e.g., air temperature, air flow direction/speed, and humidity) are maintained according to predetermined parameters based on factors such as lumber type, lumber thickness, and the starting moisture content of the lumber. The insulated chamber must be opened to remove the dried lumber and to insert the next batch of green lumber, requiring a readjustment of the temperature and other conditions within the chamber between successive batches of lumber.
Some mills have begun to use continuous kilns that include a central heating zone with a preheating/cooling zone at each end. The preheating/cooling zones are typically of equal length, and are typically 70 to 100% of the length of the central heating zone. Two parallel paths extend through the three zones. Green lumber traveling toward the drying chamber on one path is preheated by heat from dried lumber exiting the drying chamber along the other path, and by moist heated air from the drying chamber. The dried lumber exiting the heating zone is conditioned by the moisture released by the green lumber and by the moist heated air received from the drying chamber.